1. Field of the Invention
The present invention relates to a process for producing a photolithographic plate used for the graphic arts and, in more detail, to a litho-developer composition for producing a photolithograhic plate used for the graphic arts.
2. Description of the Prior Art
In order to produce a photolithographic plate used for the graphic arts, generally a litho-type photosensitive material for producing half-tone dots or line drawings or a high contrast photosensitive material by which images having very high contrast and excellent sharpness are formed have been used.
A half-tone printing plate can be produced by exposing a litho type photosensitive material to an image of the original through a contact screen and treating the material with a litho-developer composition.
It is preferable that the half-tone printing plate consist of dots having a maximum density and a background having a minimum density. However, areas having a intermedite density, the so-called fringe, are caused around the dots, because even the high contrast photosensitive material has an intermediate density. The fringe is not desirable for the graphic arts, because it causes a remarkable deterioration of the quality of the printing images. Namely, the quality of the half-tone dots is an important characteristic of a photolithographic plate used for the graphic arts.
The term litho-developer composition means an alkaline treating solution containing a known dihydroxybenzene type developing agent and an aldehyde-alkali hydrogen sulfite addition salt as a preservative (so-called infectious developer). Since a litho-developer composition of this kind includes a small amount of free sulfite ions so as to accomplish high contrast development, the stability of the developer composition is very inferior to that of the common monochromatic developer composition. Thus, an attempt for improving the stability by adding an antioxdant other than sulfite has been made. However, a developer composition having good stability as in the common monochromatic developer composition has not been obtained.
In order to treat the litho type photosensitive material, a plate development has been carried out widely hitherto. However, since plate development is complicated, a treatment using an automatic developing apparatus has been carried out recently. In treating using an automatic developing apparatus, the treating ability of the developer is kept uniform by adding a supplemental amount of the developer composition for every treatment of the photosensitive materials. However, it is necessary to adjust the quantity of the supplemental developer composition according to the size of the photosensitive material to be treated and the blackened area thereof. Where the developer composition is left as it is overnight in the plate of the automatic developing apparatus, it is necessary to add a large amount of the supplemental developer composition to recover the sensitivity thereof to the beginning level, because the stability of the developer composition is very inferior and the sensitivity deteriorates greatly. Accordingly, the treatment requires a long period of time, because the developer composition should be examined using a previously exposed control strip before treatment.
Although the sensitivity recovers due to the addition of the supplemental developer composition, the quality of the dots and the half-tone gradation related to the tone reproduction of the original do not return completely to the level obtained using the fresh developer composition.
Namely, it is impossible to retain a uniform treating ability. This tendency increases as the period of use increases. Further, if the litho-developer composition is left as it is for more than two days, the sensitivity and quality of half-tone dots do not recover at all even if a large amount of the supplemental developer composition is added, and, consequently, black spots appear. The above described situation is well known in the industry. Treatment of these photosensitive materials can be carried out only by a person who has a large amount of experience over a long period of time, and now much attention and time are required for controlling such a development.
Further, in such a treatment, materials dissolved from the photosensitive material and the oxidation products of the developer accumulate in the solution to cause the formation of scum and a deterioration of the treating ability.
Formation of half-tone dots by the litho-developer depends remarkably upon the degree of agitation of the developer composition. In general, dots of better quality can be obtained when the agitation is carried out gently. Accordingly, the quality of the dots obtained using an automatic developing apparatus is very different from that obtained using another automatic apparatus in which the character of the agitation is different. It is rarely the case that the intended quality of the photosensitive material results, because the litho-developer composition is very sensitive to pH and the developer composition has a different property each time it is prepared by the user. In actuality, even if a photosensitive material having the same quality is used, a variation results depending on the conditions and the method of treatment employed by the user.
As was described above, the prior process for treating the litho type photosensitive materials has many defects.
Accordingly, a first object of the present invention is to provide a treating process for producing a photolithographic plate used for the graphic arts which gives rise to half-tone dots having good quality.
A second object of the present invention is to provide a process for treating a litho type photosensitive material conveniently.
A third object of the present invention is to provide a treating process by which the many defects accompanying the treatment of the litho type photosensitive material by the lithodeveloper are removed and uniform properties are always obtainable.
Another object of the present invention is to provide an infectious developer composition for removing the above described defects.